New retroviral vectors are being developed for potential use in human gene therapy. Two major directions in vector development are being pursued. The first direction is the enhancement of expression and/or expansion of tropism for current retroviral vectors by replacing Moloney LTR U3 sequences with those from various other retroviral LTR's. This battery of chimeric LTR vectors will allow the selection of a vector backbone suitable for a variety of gene therapy protocols. The second major direction is the development of retroviral vectors with inducible and tissue specific expression. Recombinant LTR's containing the enhancer elements from inducible cellular genes or the TAT responsive element of HIV have been constructed. Intact Moloney LTR vectors have been built with the reporter gene CAT whose expression is driven off various cellular enhancer/promotor sequences.